Camera tube having glass membrane with layer of magnesium oxide and nickel oxide

ABSTRACT

TELEVISION CAMERA CATHODE RAY TUBES OF THE IMAGE ORTHICON TYPE HAVING A THIN, GLASS CHARGE STORING MEMBER WITH A VERY THIN LAYER OF METAL OXIDE DEPLOSITED THEREON. THE COATING OF METAL OXIDE ON THE CHARGE STORING OR TARGET MEMBER GREATLY REDUCES THE DISADVANTAGEOUS EFFECTS OF THE DEPOSITING OF ALKALI METALS ON THE CHARGE STORING MEMBER DURING THE FORMATION OF THE TUBE. THE METHOD OF APPLYING METAL OXIDE TO THE CHARGE STORING MEMBER INCLUDES THE FORMATION OF A PELLET CONTAINING A POWDERED MIX AND BOMBARDING THE PELLET WITH AN ELECTRON BEAM TO PRODUCE COATING BY EVAPORATION OF   THE PELLET IN VACUO UNTIL THE REQUIRED COATING THICKNESS IS REACHED.

United States Patent I 1 I I Priority CAMERA TUBE HAVING GLASS MEMBRANEWITH LAYER 0F MAGNESIUM OXIDE AND NICKEL OXIDE 3 Claims, 1 Drawing Fig.

US. Cl 313/651, 313/68 Int. Cl ..H0lj 31/26,

HOlj 31/36, HOlj 29/41 Field of Search 313/65, 67, 6ST

LLLZ l IO 10 i 1 l0 IO [56] References Cited UNITED STATES PATENTS2,518,434 8/1950 Lubszynski. 2,587,830 3/1952 Freeman. 3,179,834 4/1965Ochs. 3,202,854 8/1965 Ochs. 3,258,434 6/1966 Mackenzie et al 3 l 3/65T3,308,324 3/1967 Van Asselt 313/65 3,405,309 10/1968 Goetze et a1 3l3/67X FOREIGN PATENTS 879,569 10/1961 Great Britain. 313/65A PrimaryExaminer- Robert Segal Attorney- Baldwin, Wight, Diller & BrownABSTRACT: Television camera cathode ray tubes of the image orthicon typehaving a thin, glass charge storing member with a very thin layer ofmetal oxide deposited thereon. The coating of metal oxide on the chargestoring or target member greatly reduces the disadvantageous effects ofthe depositing of alkali metals on the charge storing member during theformation of the tube. The method of applying metal oxide to the chargestoring member includes the formation of a pellet containing a powderedmix and bombarding the pellet with an electron beam to produce coatingsby evaporation of the pellet in vacuo until the required coatingthickness is reached.

CAMERA Tl Bl-l H.-\\ ING GLASS \IEMBRANE WITH LAYER ()F \IAUNESIL .\lOXIDE AND NICKEL OXIDE l'his l' a streamline continuation of 580.361filed Sept. 19. I965. no abandoned.

This invention relates to ll\l\lt ll camera cathode ray tubes and morespecifically to image orthicon tubes of the kind in which a pattern ofelectric charges is produced by means of a ;hoto-cathode exposed to asubject of transmission on and stored by a thin glass film membranewhich is scanned by a scanning cathode ray to neutralise the charges anddevelop picture signals.

In manufacturing image orthicon tubes of all kinds there is employed aphoto-cathode forming exhaustion step during which considerablequantities of caesium and/or other alkali metals are vaporized insidethe envelope. The introduction of such alkali metals is required. as iswell known, in the processing of the photo-cathode material used in thetube. Inevitably some of the alkali metal condenses on to the surface ofthe target structure of the tube. In the case of an image orthicon tubeof the kind in which the target structure comprises a glass supportmember with a transparent metal signal plate (through which an opticalimage of a subject of transmission is projected) on one side and on theother side a mosaic of discrete conductive elements coated on the facesthereof remote from the glass support with photosensitive material. suchcondensed alkali metal provides partially short circuiting conductivepaths between the mosaic elements. This defect can be, as is known,overcome in this type of image orthicon tube by depositing an insulatinglayer of metallic oxide, such as magnesium oxide, directly on the faceof the glass support remote from the signal plate and forming thephoto-sensitive coated mosaic elements on this layer which is, inpractice, thick enough to reduce the light passing through to thephoto-sensitive deposits by some 50% or 60%.

In the case of an image orthicon tube of the kind to which thisinvention relates, i.e. the kind in which the target structure is notitself photo-sensitive, there is no mosaic of discrete elements or glasssupport, but instead a glass membrane which itself acts as a devicestoring a charge image, produced by a separate photo-cathode, of asubject of transmission, the deposition of caesium or other alkali metalproduces a result which though of the same general nature as thatproduced by such deposition on the target of a tube of thephotosensitised mosaic type in that it produces conductive paths throughwhich stored charges can leak, is much more serious because the alkalideposition is in direct shunt with the entire surface of the chargestoring glass membrane. Moreover there is the difficulty that themembrane itself has to receive the electron image of a subject oftransmission from the separate photo-cathode. The present inventionseeks to overcome these difficulties.

According to this invention an image orthicon tube of the kind having anelectrical charge storing member in the form of a thin glass membranewhich when the tube is in use receives a high speed electron image of asubject of transmission from a photo-cathode and is scanned by a cathoderay to develop picture signals is provided. deposited on the face ofsaid membrane towards said photo-cathode. with a very thin layer mainlyof metal oxide ofsuch small thickness as to be substantially transparentto the high speed electrons from said photo-cathode.

According to a feature of this invention an image orthicon tube of thekind having an electrical charge storing member in the form of a thinglass membrane which when the tube is in use receives a high speedelectron image of a subject of transmission from a photo-cathode and isscanned by a cathode ray to develop picture signals is provided,deposited on the face of said membrane towards said photo-cathode, witha very thin layer mainly of metal oxide of between 25 and 60 angstromsthick. Preferably the thickness of the metal oxide layer is between 30and 50 angstroms.

The deposited metal oxide is preferably mainly MgO though other caesiumresistant metals may be used.

It has been found. rather surprisingly. that even so thin a depositedlayer as one which does not seriously obstruct the high speed electronsfrom the photo-cathode-eg. a layer only (say) 40 angstroms thick-is infact sufficient greatly to reduce if not completely avoid thedisadvantageous effects which would occur from caesium deposited on theglass membrane if the oxide layer were not present to prevent it.

The thickness of the deposited oxide layer required for best results iscomparatively critical. In experimental testing with a typical imageorthicon tube of the kind referred to, excellent results were obtainedwith a deposit of MgO 40 thick. When the thickness of the deposit wasincreased to 50A or decreased to 30 A, markedly poorer results wereobtained and when the thickness increased to 60 or reduced to 25 A theresults became very much poorer.

The explanation of the success of the invention is not fully andcertainly understood. It is known however that the resistivity of a verythin deposit depends, amongst other things upon the nature of thesupport upon which the deposit is made and, while the invention is notdependent upon the correctness and sufficiency or otherwise of theexplanation now to be advanced, it is believed that the explanation ofthe substantial improvement which in fact the invention achieves is thatthe resistivity of the deposit of alkali metal produced as an inevitableconcomitant of the vaporization of that metal during the exhausting andphoto-cathode forming step of manufacture, is substantially higher whenthe metal is deposited on the very thin layer of metal oxide on theglass membrane than it would be if it were deposited directly on thatmembrane.

A metal oxide layer of the range of thickness above stated is easilypenetrated by the high velocity electrons from the photo-cathode of animage orthicon tube of the kind to which the invention relates. It is,however, generally desirable to provide a layer of metal oxide on bothfacesof the glass membrane but, if this is done, it is not satisfactorytontake the layer entirely of MgO because such a layer, present on theside of the membrane facing towards the scanning electron gun, would be,although thin, not sufficiently transparent to the relatively slow speedelectrons from the said gun and severe sticking would result.

Preferably therefore, in carrying out this invention, the glass membraneis provided on both sides with a layer consisting mainly of MgO to whicha small proportion of metal oxide of definite but limited conductivityhas been added. A preferred composition for a layer provided on bothsides of the glass membrane is approximately by weight of MgO andapproximately 10% by weight of conductive oxide, preferably MO. Theproportions of MgO and conductive metal oxide may be varied betweenfairly wide limits to suit requirements but preferably the limits are 5%and 33.3% by weight of the conductive metal oxide. NiO is the at presentpreferred conductive oxide, but an oxide of Cr, Mn, Fe or C0 may beused, or there may be employed a mixture selected from the oxides of Cr,Mn, Fe, Co and Ni.

A preferred method of carrying out the present invention is as follows:

(1) Carbonates of Mg and Ni (or whatever metal or metals is or are to beused to provide the conductive oxide to be mixed with MgO) are weighedout in the required proportions and intimately mixed by grinding in aball mill;

(2) The ball-milled powder is pressed into pellets of suitable shape andsize. Disc-like pellets about /2" diameter and he" thickness areconvenient. The pellets are fired at about 1000C at which temperaturethe carbonates are broken down to oxides and a rather soft ceramicresults;

(3) Glass membranes to be treated are coated in vacuo with the oxidemixture by evaporation produced by bombarding a sintered pellet by anelectron beam focussed thereon. The thickness of the layer thus producedon the membrane by evaporation is monitored durmg the maporauon step. bat the same time also depositing the emporated material on to a quart!crystal the frequency of which I\ COrlliflUUttsl} obsened. l'heevaporation step is halted when the cry stal reaches a predeterminedfrequency found. by previous trial and error experiment. to be reachedwhen deposition has continued to the point at which a layer of therequired thickness has been obtained on the membranes under treatment:and

(4) The treated glass membranes are mounted in their respective tubesand the normal known processes of tube manufacture. including theexhaustion step with vaporization of alkali metal to form thephoto-cathode. are continued.

In the drawings:

FIG. 1 is a schematic illustration of an image orthicon tube having acharge storing membrane to be treated in accordance with the presentinvention; and

FIG. 2 is a schematic illustration of the image section of an imageorthicon tube and shows a glass. Charge storing membrane having layersof metal oxide coated thereon.

The image orthicon tube of FIG. 1 consists of an evacuated envelope 1having a photo-cathode 3 mounted on the end wall of the envelope 1. Theimage orthicon tube further includes a glass membrane target or chargestoring member 4. a field terminating mesh 7. a wall anode 8. anelectron gun 9. and a dynode system 10. The image orthicon tube of FIG.1 is exemplary of the type of tube into which the subject matter of thepresent invention may be incorporated and it is not intended in any wayto be definitive of all such tubes incorporating the subject matter ofthe present invention.

The image section end of an image orthicon tube embodying the presentinvention is more clearly shown in FIG. 2. The tube comprises the glassenvelope 1 of enlarged diameter over the image section 2 enclosing thephoto-cathode 3. Spaced from the photo-cathode 3 is the thin glassmembrane target 4 and interposed between the target 4 and thephotocathode 3 is the customary screen 5 and accelerating elec trode 6.As so far described. the tube is as well known per se. In accordancewith the present invention the thin glass membrane target 4 hasdeposited on both sides. to a thick ness of 40 angstroms, a layer havinga composition of approximately by weight of magnesium oxide and 10% byweight of nickel oxide.

Image orthicons of the kind referred to and in accordance with thisinvention have been found experimentally not only to have materiallybetter resolution than comparable known tubes but to manifest asignificant improvement in transfer characteristic. there being littleor none of the undesirable over-exposure effect known as white-crushing"(loss of discrimination in the brighter picture areas) so oftenexhibited by known image orthicons as at present in common use. Alsosuch improved tubes have improved sensitivity and there are indicationsthat the signal-to-noise ratio is somewhat improved.

We claim:

I. An image orthicon tube including a photocathode; an electrical chargestoring member in the form of a thin glass membrane, said member beingso positioned that when the tube is in operation it receives an electronimage of a subject of transmission from said photocathode; scanningmeans including an electron gun and electrodes to which acceleratingpotentials may be applied adapted and positioned to scan said memberwith a cathode ray to develop picture signals characterized in that saidelectrical charge storing member consists of a thin glass membranehaving deposited on the face thereof towards said photocathode a verythin layer of magnesium oxide containing from about 5 to 33.3 percent byweight of nickel oxide, said layer having a thickness of between 25 and60 angstroms and being of such small thickness as to be substantiallytransparent to the electrons emitted by said photocathode.

2. A tube as claimed in claim 1 wherein the layer is between 30 and 50angstroms thick.

3. A tube as claimed in claim 1 wherein the layer is deposited on bothsides of the membrane.

